1. Field of the Invention
The present invention relates generally to rail fasteners, and more particularly, but not by way of limitation, to an improved rail tie fastening assembly for fastening a rail to a concrete tie and effectively transferring applied loads from the rail to the tie.
2. Brief Description of Related Art
Pretensioned, prestressed concrete railroad ties have evolved since the early 1940's into a proven mechanism to attach railroad rails together, maintain track gauge, and transfer wheel loads to the ground. The base material of these ties is Portland cement concrete reinforced with high strength steel wires which are pretensioned prior to casting to maintain the concrete in compression and thus prevent cracking. The high strength concrete used (8,000 psi or greater ultimate compressive strength) is a stiff, brittle material. Metal fasteners, designed to hold the steel rail to the concrete tie, are part of the mechanism used to transfer applied wheel loads to the ballast.
Presently, two types of fasteners are generally used to fasten rails to concrete ties. The first type of fastener is a positive hold down device which can take a variety of forms such as screws or bolts used together with some form of flanged clip to hold the rail base flange in contact with the tie. This type of fastener is rigid and thus prone to fatigue failure in service, and therefore, is not commonly employed today.
The second type of fastener is in the form of a spring fastener, also used to hold the rail flange in contact with the tie, but designed to reduce fatigue from applied loads by flexing. In the spring type fastener, two iron or steel shoulders are embedded in the concrete tie at each rail seat during casting and serve to hold the rails in gauge and to anchor the spring clip which in turn holds the rail flange down. These spring clips are designed to apply a known vertical force to the rail flange to resist rail uplift between wheel passes and to transfer longitudinal forces from temperature change or train acceleration/deceleration to the tie and into the ground.
In conjunction with the fastener, an elastomeric pad approximately six inches square and one-quarter inch thick is installed between rail and the top of the tie at the seat to accommodate the differences in surface form. If the rail flange were to bear directly upon the concrete tie surface, the steel would soon wear into the top of the concrete. Although the tie is cast in a steel mold, the seat surface does not conform exactly with the rail flange bottom resulting in the potential for point loading and uneven vertical force transfer.
These pads also perform two additional functions. First, because the rail is clamped tightly to the tie by spring clips, the pad, which has a higher coefficient of friction than steel on steel, helps transfer longitudinal forces along the rail into the tie and ballast. Second, and more important, the pads serve to attenuate shock loads applied to the rail by flat spots on passing steel wheels. Shock loads from wheel flats may be two to four times the amplitude of normal wheel loads, and of very short duration, typically about 15 milliseconds. These shock loads tend to fracture the concrete tie if not properly attenuated.
A number of problems have been encountered with the spring type positive retention fastener. The first problem is pad retention. Pad retention is concerned with holding the pad in place under the rail between the rail flange and the tie when the rail is flexed with applied wheel loads. Various shapes have been used to try to keep the pad from working out by retaining it mechanically. This has been only moderately successful, particularly on curved tracks which is the principal location of concrete ties in the United States. Alternatively, pads of varying hardness have been used. However, pads resilient enough to attenuate shock loads, tend to work out from between the rail and the tie under normal wheel pass cycles. Harder pads do not satisfactorily attenuate the shocks.
Another solution to unwanted pad movement has been to adhere the pad to the concrete tie surface with adhesive. This holds the pad in place, but makes replacement difficult when the top surface of the pad wears out. Also, applying an adhesive in the field to a wet, dirty tie surface presents problems with the quality of the adhesion.
Another problem encountered with the use of shock pads has been rail seat abrasion. Dirt and grit from the field tend to work between the pads and concrete tie surface. Sanders on locomotive wheels, used for traction enhancement, also work sand under the pads. When water supplied by rain is applied to this mix, a grinding compound is formed which works to abrade the concrete surface under passing wheel loads. Adhered seat pads help to alleviate this problem but have the same down side of field replacement when worn. Various metal/elastomer pad combinations have been tried with some success to get rid of rail seat abrasion, but tend to be expensive, particularly when field applied.
Finally, tie lift caused by rail uplift between wheels has been a problem. Tie lift is inherent to all positive fixation fasteners, both screw and spring type. In order for the fastener to work, the rail flange must be held tightly to the tie surface. Since a rail is a continuous beam on multiple flexible supports, it deflects downward under the passing wheel, and the rail between wheels deflects upward from its normal or rest position. The uplift force of the rail is often greater than the weight to the rail/tie assembly. Therefore, with the positive fixation fastener, the tie also is lifted from the surface of the road bed between each wheel set and then forced back onto the ground by the next wheel. This repetitive tamping action quickly damages the roadbed ballast.
It is the resolution of the above mentioned problems that the present invention is directed.